1. Field of the Invention
The present invention concerns the integration of one or more ROM (Read Only Memory) memory cells in a matrix of electrically programmable non-volatile memory cells, said matrix formed using of a Self-Aligned Source (SAS) process.
2. Discussion of the Related Art
Several devices include matrices of programmable non-volatile memory cells, typically EPROM, EEPROM or Flash EEPROM memory cells. Such devices are for example stand-alone memories, or microcontrollers with an embedded non-volatile memory. It can be necessary to include ROM memory cells in the matrix of electrically programmable memory cells. The ROM portion of the matrix is used for permanently storing parts of a program or codes which are not to be modified by the user.
ROM memory cells are normally formed by transistors whose xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d logic content is programmed xe2x80x9chardwarexe2x80x9d during the manufacturing process, so that the content cannot be electrically altered.
Conventionally, a ROM memory cell is formed by a MOS transistor, e.g. an N-channel one, having N type drain region and source region formed spaced-apart in a P type substrate or well. The substrate portion between the drain and source regions forms a channel region, and a gate electrode is superimposed over the channel region by the interposition of a thin gate oxide layer.
Typical methods used for setting the logic content in a MOS transistor during the manufacturing process are:
1. Active area programming: in FIG. 1 there is schematically shown a top-plan view of some ROM memory cells arranged in rows and columns to form the matrix structure of a semiconductor memory device. There is shown an N+ common source line 1, N+ drain regions 4 formed using implantation and diffusion, two polysilicon word lines 2, typically formed using a lithographic process, which run parallel to the common source line and correspond to the gate regions of the cells, and some field oxide regions 3, defined in an early process step by means of a conventional selective thermal oxidation (xe2x80x9cLOCOSxe2x80x9d). The field oxide regions 3 bound the active areas of the device. The low logic level (xe2x80x9c0xe2x80x9d) (cell 6 in FIG. 1) is obtained by interrupting the electrical connection of the active area, using a suitable layout of the photolithographic mask used for defining the active area. A suitable dimension d for the interruption of the active area must be respected to assure that the electrical connection is effectively interrupted. In this way, a memory cell 6 is obtained including a MOS transistor whose gate oxide is the thick field oxide. This MOS transistor thus has a threshold voltage much higher than a conventional MOS transistor, and additionally the source and the drain are spaced apart from the gate electrode (word line 2). The high logic state (xe2x80x9c1xe2x80x9d) is achieved by forming a conventional MOS transistor 5. Programming the memory cells is thus performed at the level of the mask for forming the active areas of the memory device, and no further masks are required.
2. Programming by P-LDD doping: similarly to FIG. 1, FIG. 2 schematically shows a top-plan view of some cells of a semiconductor memory device. For programming the low logic state (xe2x80x9c0xe2x80x9d) in the memory cell 7, a P-LDD mask 8 is used. Since ROM memory cells are typically formed by N-channel MOS transistors, the LDD implant for P-channel MOSFETs is used for counterdoping by a P type dopant the LDD region of the N-channel transistor. Opening the P-LDD mask 8 over the N-channel MOSFET forming the memory cell 7, a transistor is obtained with detached junctions. In such a way the electrical connection between source and drain is interrupted and the transistor cannot be driven by biasing the gate. A memory cell storing the low logic state is thus obtained. The high logic level (xe2x80x9c1xe2x80x9d) is obtained as usual by forming a conventional transistor. Also in this case, programming of a cell is done by modifying one mask, during the manufacturing of the device, without the necessity of additional masks.
3. Programming using threshold-voltage regulation implant: the low logic state can be achieved by using a suitable P type doping, used for regulating the threshold voltage of the N-channel MOS transistor forming the cell to be programmed. The doping affects the channel of the transistor. Such a method is for example described in U.S. Pat. No. 5,403,764. In this way it is possible to obtain transistors with as high a threshold voltage as desired. A drawback is that the breakdown voltage of the drain junction is lowered, due to the increased dopant concentration gradient between the drain junction and the channel. Furthermore, an additional mask is required for the formation of transistors storing the low logic level, i.e., having a high threshold voltage.
The demand for semiconductor memory devices with high-density memory matrices has made it necessary to reduce the size of the elementary memory cells. One of the techniques allowing such a reduction is the Self Aligned Source (SAS) process, described for example in U.S. Pat. No. 5,103,274. The difference between a SAS process and a conventional, non-SAS process is in the layout of the active area 9 in the matrix which, as shown in FIGS. 3 and 4, has no distinctions between the source and drain regions. In FIGS. 3 and 4, some memory cells of a matrix of a semiconductor memory device are shown: in FIG. 3 the active area 9 of a non-SAS matrix has the conventional T shape. In FIG. 4 the active area of a SAS matrix is shown, at a step preceding the SAS etching by means of the SAS mask: the typical stripe layout of the active area is visible. Subsequently, the field oxide regions 3 are completely removed from the stripe included between the two word lines 2.
By means of the SAS process a memory device is obtained having the edges of the field oxide regions vertically aligned to the polysilicon word lines. The source regions, formed between the edges of the field oxide regions of adjacent cells, are thus self-aligned to both the field oxide regions and the word lines.
The self-alignment of the source regions allows a closer distribution of the polysilicon word lines, without any decrease in the source width. Thus, a lower distance between one memory cell and the adjacent one is required. The size of the elementary memory cell is thus reduced, and a higher density of the overall device can be achieved.
The conventional, non-SAS technique is not practicable for devices requiring a high capacity, since the area of the memory could impact significantly the physical dimensions of the device, making it inconvenient to be industrially produced.
On the other hand, forming ROM memory cells using the method of programming by P-LDD doping (point 2 above) or using the method of programming by ion implantation into the channel for regulating the threshold voltage (point 3 above) have the following shortcomings:
they require the use of an additional mask, as well as additional process steps;
they produce a lowering of the breakdown voltage of the drain junction, not compatible with the operating voltages of the EPROM or Flash EEPROM devices wherein the ROM memory cell is integrated as a portion of the matrix.
In view of the state of the art described, the present invention provides a semiconductor memory device comprising ROM memory cells inside a matrix of memory cells formed by means of a Self-Aligned Source process, without incurring in the above-mentioned drawbacks.
According to the present invention, this and other advantages are achieved by a matrix of memory cells formed using a method allowing for a self-alignment of the respective source region with the respective field oxide layer and the respective overlying polysilicon layer of each single cell of the matrix, comprising at least one first ROM memory cell suitable for permanently storing a first logic level, associated with a respective row and a respective column of the matrix, said first cell comprising a silicon substrate of a first conductivity type over which a first and a second isolation region are formed, delimiting therebetween a longitudinal stripe, a gate element extending transversally through the stripe from at least one side of the first isolation region to at least one side of the second isolation region, a third region of a second conductivity type and a fourth region of a second conductivity type formed in the substrate along the stripe, and means adapted to prevent the formation of a conductive channel in the substrate, and at least a second ROM cell for permanently storing a second logic level, identical to the first ROM memory cell but not provided with said means.
Still according to the present invention, there is provided a process for manufacturing a matrix of memory cells, comprising a first step of definition of first, parallel active area stripes delimited by field oxide regions, a second step of definition of rows of the matrix extending transversally to the stripes, a third step of removal of the field oxide regions between pairs of alternated rows for obtaining, between said pairs of alternated rows, second stripes of exposed substrate, delimited by adjacent edges of the rows of said pairs, a fourth step of implantation for the formation of source and drain regions, wherein, in the first step, there is provided a layout of the mask for the definition of the active area suitable to allow the presence of oxide regions in the areas wherein ROM memory cells are provided suitable for permanently storing the first logic level.
The process according to the invention allows for integrating ROM memory cells in a matrix of electrically programmable memory cells formed according to the SAS method. Additionally, the process according to the invention allows switching from the production of an EPROM or Flash EEPROM version of a device to a ROM version of the device, with minimum process changes and leaves most of the masks unchanged.